You are correct - it did take about a year to get the legislation passed to create the "Consolidation Warehouses". Dave's date could simply be the year the whiskey went into the consolidation warehouse from where the whiskey was stored before. If so it was peobably a non bonded blend. It should be also noted that the DSP No. for whiskey is the distillery where the whiskey goes into the barrel, not necessarily where it was made. When I was at U.D. Brown-Forman made some I W Harper while the Bernheim distillery was being built. It was shipped in bulk to Bernheim and put into the barrel at Bernheim with DSP 1 on the barrel. Something along those lines could be the cause of Daves tax stamp - the whiskey was made somewhere else and when the barrels finally arrived at a bonded warehouse, they were entered into the books for 1921.
It should be remembered that before prohibition over half of the whiskey on the market was a blend of some kind. These blends were sold in both barrels and bottles and some blenders created a consistant taste for their bottled product by aging their blends after blending for six months to a year in barrels. If you had a blender who sold 20,000 barrels a year but decided in 1919 with prohibition coming and people stocking up for the dry times ahead, he will produce 30,000 barrels. He did a fair estimate of his business but still only sold 28,000 barrels and has 2,000 barrels on hand when prohibition came into effect. He holds these barrels in his warehouse because they are his property even if he can't sell them to the public. After a year the government makes him place his 2,000 barrels into a bonded, consolidation warehouse and it is given a date of 1921 and the DSP Number for the distillery of the warehouse. Later he sells this whiskey to someone who owns a license for one of their products, or writes a contract with one of these companies to bottle and sell it for him giving them a price for each bottle sold - usually about 5 cents a bottle plus bottling cost.